Offset parabolic reflector antenna

ABSTRACT

In accordance with the disclosure, two or more main beams of an offset parabolic reflector antenna are directed without producing second order aberrations by placing their respective feeds so that their phase centers reside at mathematically defined locations.

United States Patent [151 3,696,435

Zucker 1 Oct. 3, 1972 [54] OFFSET PARABOLIC REFLECTOR [56] References Cited ANTENNA UNITED STATES PATENTS [72] Inventor: Henry Zucker, Parslppany-Troy Hills Township, Mon-is County NJ 3,569,976 3/1971 Korvm ..343/779 [73] Assignee: Bell Telephone Lalioratories, lncorprimary Examiner E1i Lieberman porated, Murray Hill, NJ. AttorneyR. J. Guenther and William L. Keefauver [22] Filed: Nov. 24, 1970 [57] ABSTRACT [21] Appl. No.2 92,400

In accordance with the disclosure, two or more mam beams of an offset parabolic reflector antenna are [52] US. Cl. ..343/779, 343/840, 343/854 directed without producing second order aberrations [51] Int. Cl. ..H0lq 19/ 14 by placing their respective feeds so that their phase I Field of Search "343F776, 6 8 5 4 centers reside at mathematically defined locations.

3 Claims, 3 Drawing Figures EDGE OF FOCAL PLANE I4 DIRECTION OF MAIN BEAMS FOR F AND F2 FOCAL POINT I2 EDGE OF PLANE l6 OFFSET PARABOLIC REFLECTOR ANTENNA BACKGROUND OF THE INVENTION 1 Field of the Invention This invention relates to offset parabolic reflector antennas.

2. Description of the Prior Art An offset antenna has an advantage over a symmetrical antenna in that blocking of the antenna aperture by the antenna feed is not present. Elimination of such blockage results in better control of unwanted sidelobes when transmitting or receiving and, when transmitting, of unwanted radiation back into the feed. These advantages are discussed in U.S. Pat. No. 2,644,092, issued to J. R. Risser on June 30, 1953; U.S. Pat. No. 3,407,404, issued to J. S. Cook et al. on Oct. 22, 1968; and U.S. Pat. No. 3,500,427, issued to S. Landesman et al. on Mar. 10, 1970.

Each of the above-mentioned patents discloses an offset parabolic reflector having a single feed and a radiation pattern having a single main beam. Antennas having multiple feeds to provide multiple independent main beams are, on the other hand, sometimes desirable. U.S. Pat. No. 3,406,401, issued to L. C. Tillotson on Oct. 15, 1968, for example, shows the use of a multiple feed, spherical reflector antenna for satellite communication use. To the best of applicants knowledge, however, a low aberration multiple feed antenna which includes the advantages offered by the offset parabolic reflector antenna in the on-axis position has not been available.

SUMMARY OF THE INVENTION An object of the invention is to provide multiple feeds in an offset parabolic reflector antenna in such a manner as to produce multiple main beams having relatively low aberration levels.

This and other objects of the invention are achieved in an offset parabolic reflector antenna by locating each of two or more feeds so that their respective phase centers reside at mathematically defined locations with respect to the offset parabolic reflector. In particular, multiple main beams having substantial zero second order aberrations are achieved by locating two or more feeds so that their phase centers reside substantially on a line unique to the particular reflector.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 shows a side-elevation view of an embodiment of the invention; and

FIG. 2 and 3 show end-elevation and top views, respectively, of the embodiment.

DESCRIPTION OF THE DISCLOSED EMBODIMENT The side-elevation view of Fig. 1 includes a reflector 10. This reflector comprises an off-axis sector of a paraboloid of revolution formed by the intersection of the paraboloid and a right angle cone whose vertex is located at the focal point of the paraboloid. In FIG. 1 the outline of the paraboloid of which reflector is a sector is shown by a broken line 11 while its focal point and vertex are identified by symbols 12 and 13, respectively. In the end-elevation view shown in FIG. 2,

reflector 10 has a circular projection. In the top view shown in FIG. 3, reflector 10 has an elliptical projection. Although an off-axis sector formed in this manner has advantages from a radiation pattern standpoint, offset sectors of other configurations may be used in practicing the invention.

Reflector l0 and subsequently mentioned feeds may be positionally related through the use of three reference planes. The first plane is a focal plane 14 which includes focal point 12 and, furthermore, is perpendicular to the paraboloid line of symmetry which passes through focal point 12 and vertex 13. The second of the planes is a symmetrical plane 15 which includes the paraboloid line of symmetry and, furthermore, bisects reflector 10. The last of the three planes is referred to as asymmetrical plane 16. This plane passes through focal point 12 and is mutually perpendicular to the other planes.

Two feeds F and elevation F are schematically represented by a pair of dots in the drawing. These feeds may comprise horns or other conventional feeds directed so as to subtend solid angles with reflector 10. Furthermore, because of the reciprocal nature of antennas, they may be used for either transmitting or receiving.

Feeds F and F are positioned so that their phase centers are displaced to the vertex side of focal plane 14 by perpendicularly measured distances substantially equal to Z and Z respectively, and to either side of symmetrical plane 15 by perpendicular distances substantially equal to x, and x respectively. On the other hand, the phase centers lie substantially in asymmetrical plane 16. Furthermore, feed F subtends a substantially solid angle with reflector 10. As shown in FIG. 3, the direction of the main beam for feed F makes an angle [3, with respect to the paraboloid line of symmetry. Similar but unidentified angles having a subscript of two applied thereto relate to feed F In accordance with the invention, second order aberrations in the radiation patterns are substantially eliminated when the values of x and z for each feed are substantially defined by the following:

x=mfsina 1 z=f(mcosa-l) 2 where f is the distance from focal point 12 to vertex 13, and

m and a satisfy the expressions:

sin A 2 2 |:1+( A+cos B 1 +2 [1-m cos a] sin A cos A-l-cos B msina 2 sin A 2 2 (cos A-l-cos B "m cos sin A 2 (cos A+0os B) (I'm cos sin A 2+ (cos (m sin (1) A+cos B (The above relationships are derivable from a spherical coordinate system centered about a point located on the paraboloid line of symmetry and at two focal lengths from the vertex, where the focal length is, of course, the distance between focal point 12 and vertex 13. The angle a is the angle between the line of symmetry and the line drawn between the coordinate system center and the phase center of the feed. Furthermore, m is the ratio of the distance of the phase center from the coordinate system center to the focal distance.)

An embodiment of the invention for operation at 16 Gl-lz was constructed and successfully operated. The embodiment parameters are:

Antenna aperture diameter 48.4%, Focal length 40.0A, Angle A 47.5, and Feeds displaced to either side of the line of symmetry by a 4.2",

where) equals the wavelength at l6 GHz.

Measurements showed that the main beams produced by the feeds were displaced to either side of the line of symmetry by ,8 3.4. There was some broadening of the beams, mainly below the 10 db level, and also higher first sidelobes. However, these sidelobes were at the 30 db level. For all practical purposes, there was no significant increase in sidelobe levels for off-axis positions in comparison to the onaxis, focal-point position.

It should be noted that scanning is possible by moving feeds F and F through an arc defined by a continuous set of values for x and z. If only one main beam is desired, one of the feeds may be eliminated.

Further appreciation of the invention may be obtained by selecting the values of angles A and B so that:

sin A/(cosA+cosB)=l 5 When this is done, expressions (3) and (4) become:

sin 5: m sin a {Him-2m cos (6) and O +3m c S 7+m m 7 Ratio m therefore determines the values of angle a and angle B as follows:

m a B L000 0 0 1.001 1.810 0.906 L005 4.040 2.024 L010 5.696 2.869 1.015 6.956 3.5l7 1.020 8.010 4.065 [.025 8.930 4.548 1.030 9.754 4.986 L035 10.500 5.300

What is claimed is: l. A directive antenna for producing at least two main beams, said antenna comprising a reflector which is an off-axis sector of a paraboloid of revolution having a focal length f, and

at least two feeds where each feed is directed to subtend a solid angle with said sector and its phase center is located at a point substantially defined by x=mfsin a,

y=0, and

sin A 2 2 (cos A+cos B)] +2 [1m cos a] [2-( 212 Y] m sln 011 and sin A cos A+c0s B sin A 2 (cos A+c0s B) (1%? cos (m sin a) i sin A 1m cos a) 2+ cos A+cos B m cos a 0 where:

B the off-axis steering angle between the main beam related to that feed and the line of symmetry of said paraboloid,

B one-half of the angle sustained in said second plane by said reflector with said focal point, and

A the angle formed by a line which bisects said lastmentioned angle and said line of symmetry.

2. A directive antenna for producing at least two main beams, said antenna comprising a reflector which is an off-axis sector of a paraboloid of revolution having a focal length f,

said sector definable with respect to three planes where the first plane is the focal plane of said paraboloid, the second plane includes the focal point of said paraboloid and bisects said sector, and the third plane passes through said focal point and is mutually perpendicular to the other two planes, and

at least two feeds where each feed is directed to make a solid angle with said sector and is located so that its phase center is substantially in said third plane at a point substantially defined by x=mfsina where:

x a distance perpendicular to said second plane and measured from either side thereof, z= a distance perpendicular to said focal plane and measured only from the sector side thereof, and m and a satisfy the expressions sin A 2 z I (m 1+ m cos a sin A sin [3 and I sin A 2 2 (EBQ'I4 F cos B m cos sin A 2 g (W) -1" cos (m sin (1) sin A (1m cos a) 2+ cos A+cos B m cos a where:

where the first plane is the focal plane of said paraboloid, the second plane includes the focal point of said paraboloid, the second plane includes the focal point of said paraboloid and bisects said sector, and the third plane passes through said focal point and is mutually perpendicular to the other two planes, and

at at least one feed which is directed to make a solid angle with said sector and is movable so that its phase center may be moved substantially in said third plane along an are substantially defined by x mf sin a z=f(m cosal) where:

x a distance perpendicular to said second plane and measured from either side thereof, z a distance perpendicular to said focal plane and measured only from the sector side thereof, and m and a satisfy the expressions msina 2 1 sinB] and y sin A 2 2 (cos A+cos B m cos sin A cos A+c0s B (m sin (2) sin A 2 l T (1-m cos a) 2+ cos A-l-cos B m cos a 0 2 (1m cos 00] where:

B the off-axis steering angle for the main beam with respect to the line of symmetry of said paraboloid,

B one-half of the angle sustained in said second plane by said reflector with said focal point, and

A the angle formed by a line which bisects said lastmentioned angle and said line of symmetry. 

1. A directive antenna for producing at least two main beams, said antenna comprising a reflector which is an off-axis sector of a paraboloid of revolution having a focal length f, and at least two feeds where each feed is directed to subtend a solid angle with said sector and its phase center is located at a point substantially defined by x mf sin Alpha , y 0, and z f (m cos Alpha - 1) where z a distance perpendicular to the focal plane of said paraboloid and measured only from the sector side thereof, x a distance perpendicular to a second plane and measured from either side thereof where said second plane includes the focal point of said paraboloid and bisects said sector, y a distance perpendicular to a third plane where said third plane passes through said focal point and is mutually perpendicular to the other two planes, and m and Alpha satisfy the expressions
 2. A directive antenna for producing at least two main beams, said antenna comprising a reflector which is an off-axis sector of a paraboloid of revolution having a focal length f, said sector definable with respect to three planes where the first plane is the focal plane of said paraboloid, the second plane includes the focal point of said paraboloid and bisects said sector, and the third plane passes through said focal point and is mutually perpendicular to the other two planes, and at least two feeds where each feed is directed to make a solid angle with said sector and is located so that its phase center is substantially in said third plane at a point substantially defined by x mf sin Alpha z f (m cos Alpha - 1) where: x a distance perpendicular to said second plane and measured from either side thereof, z a distance perpendicular to said focal plane and measured only from the sector side thereof, and m and Alpha satisfy the expressions
 3. A scanning antenna comprising a reflector which is an oFf-axis sector of a paraboloid of revolution having a focal length f, said sector definable with respect to three planes where the first plane is the focal plane of said paraboloid, the second plane includes the focal point of said paraboloid, the second plane includes the focal point of said paraboloid and bisects said sector, and the third plane passes through said focal point and is mutually perpendicular to the other two planes, and at at least one feed which is directed to make a solid angle with said sector and is movable so that its phase center may be moved substantially in said third plane along an arc substantially defined by x mf sin Alpha z f(m cos Alpha - 1) where: x a distance perpendicular to said second plane and measured from either side thereof, z a distance perpendicular to said focal plane and measured only from the sector side thereof, and m and Alpha satisfy the expressions 